The phenomenon of luminescence, either chemiluminescence (CL) or bioluminescence (BL), is increasingly being exploited in quantitative determinations of a variety of analytes. For instance, the bioluminescent signatures of planktonic organisms have been studied in efforts to determine the species responsible for bioluminescence [U.S. Pat. No. 4,563,331].
Recently, methods for quantitating an analyte in an immunoassay protocol have been proposed which utilize luminescence detection. Such luminescence immunoassays (LIA) offer the potential of combining the reaction specificity of immunospecific antibodies with the high sensitivity available through light detection. The specificity and sensitivity of LIA reagents is generally comparable to those employing traditional radiolabelling. However, the nontoxic nature of LIA reagents and the longer shelf lives of LIA reagents relative to radioactive reagents makes LIA a generally preferred analytical method for many applications.
One exemplary bioluminescent reaction involves firefly luciferase, which mediates the conversion of luciferin to oxidation products and light. Assays of ATP using the firefly reaction have been reported to be linear over six orders of magnitude with a detection limit of 10.sup.-4 picomol of ATP [U.S. Pat. No. 4,772,453].
Not surprisingly, much interest has evolved in developing improved instruments for measuring luminescence from biological samples. Since many samples are frequently screened concurrently, e.g., for immunoreaction of a bioluminescent antibody with a target analyte, low light levels from external light sources or adjacent samples can be problematic. The instruments designed to measure the low light levels associated with luminescence are frequently referred to herein as luminometers.
Some previously proposed luminometers include those described in U.S. Pat. No. 4,772,453; U.S. Pat. No. 4,366,118; and EP 0025350. U.S. Pat. No. 4,772,453 describes a luminometer having a fixed photodetector positioned above a platform carrying a plurality of sample cells. Each cell is positioned in turn under an aperture through which light from the sample is directed to the photodetector. U.S. Pat. No. 4,366,118 describes a luminometer in which light emitted from a linear array of samples is detected laterally instead of above the sample. Finally, EP 0025350 describes a luminometer in which light emitted through the bottom of a sample well is detected by a movable photodetector array positioned underneath the wells.
Further refinements of luminometers have been proposed in which a liquid injection system for initiating the luminescence reaction just prior to detection is employed [EP 0025350]. Also, a temperature control mechanism has been proposed for use in a luminometer [U.S. Pat. No. 4,099,920]. Control of the temperature of luminescent samples may be important, for example, when it is desired to incubate the samples at an elevated temperature.
However, none of the above devices effectively avoids detection of light emitted by samples in adjacent sample wells, i.e., those not intended to be monitored when a measurement is made on a given sample. Hence, the previous devices can fail to accurately determine the light level emitted by a weakly emitting sample, particularly when it is adjacent one or more strongly emitting sample(s). This phenomenon of light measurement interference by adjacent samples is frequently referred to herein as "crosstalk".